Can you sell unused furniture?

The discount is the part that matters. You do not get back what you paid, so a piece you buy and then change your mind about costs you something permanently. Decorating is cheaper if you commit to a layout first and buy for it, rather than buying broadly and clearing out the leftovers later.

There are two places to do it, and the second one is the convenient one. Co-op vendors will take furniture back, and so will the robot shops standing on your home plot, which means you can offload a piece where you were already standing instead of travelling to a shop. What comes back is resources rather than a flat cash refund, and Aniimo runs several separate currencies, so look at what a single piece actually returns before you sell off a whole set of them.

Not settled: How steep the buyback discount is has not been pinned down, and furniture returns resources rather than a stated amount of one currency, so the value of a sale varies by piece.